Uniform illumination across custom shapes and sizes is particularly difficult to achieve for a number of reasons. Incandescent and fluorescent light sources come in fixed sizes, limiting the granularity of the illumination sources—for example fluorescent tubes come in fixed lengths and cannot be cut to length. Lighting systems based on light emitting diodes (LEDs) typically are mounted on a circuit board having an electrical topology not amenable to being cut to length. For example, for large-area lighting applications, LEDs may be electrically laid out in groups of series-connected strings, e.g., on a square or rectangular tile, where each string contains multiple series-connected LEDs all operating at the same current. While in some topologies, one or more strings may be removed to permit dimensional customization, parts of a string typically cannot be removed without opening the circuit and causing de-energization of that string. The physical layout of the string therefore may limit the level of achievable granularity. Once installed in a lighting system, such fixed size illumination sources may visually result in regions having undesirably different light intensity levels or colors. A second issue with removing LED strings is that such systems are typically driven by a constant-current driver, so when one or more strings are removed, the current from the driver is divided among fewer strings, resulting in a local brightness increase. The lack of granularity in the sizing of the illumination sources and/or possible current variations between LEDs or groups of LEDs may result in visually distinguishable variations in light intensity level and/or color, for example correlated color temperature (CCT). From an application perspective, this is undesirable because the illumination level is desirably uniform over the entire illuminated area.
Another electrical topology that may be utilized for LED-based illumination sources is the connection of all of the LEDs in parallel. This topology may permit removal of individual LEDs and thus may achieve relatively finer granularity, in some cases on the order of the LED spacing. However, such systems are prone to “current hogging,” in which the current preferentially flows through the LED(s) with the lowest forward voltage. This can result in increased heating of such LEDs, which further reduces the forward voltage, thus increasing the current-this process can continue until those LEDs fail. In some cases, this process may occur over and over, for example cascading from one LED to the LED having the next lowest forward voltage in the system. In some cases, this effect may be mitigated by carefully matching the forward voltage of all of the LEDs, but this typically adds significant expense.
Another approach is to incorporate a ballast resistor or other current-limiting device with each LED; however, this may increase cost and significantly reduce efficiency because of the power loss in the ballast resistor. A further efficiency disadvantage of this electrical topology is that it typically is driven at about the forward voltage of one LED. Low-voltage systems typically have increased power losses in the lines (wires) as well as lower driver efficiency.
A third electrical topology, using a constant-voltage supply in combination with an array, for example a parallel array, of small, low-cost LEDs configured in strings of series-connected LEDs, where each string also includes a current-regulating element, addresses a number of the deficiencies of the systems described above. Exemplary electrical and physical schematics of this approach are described in detail in U.S. patent application Ser. No. 13/799,807, filed on Mar. 13, 2013, (the ′807 application) and U.S. patent application Ser. No. 13/970,027, filed on Aug. 19, 2013 (the ′027 application).